What does Matthew 26:74 mean?
Peter told Jesus he would die before ever denying him (Matthew 26:30–35). In that spirit, he had pulled a sword and attacked those coming to arrest Jesus earlier that night (Matthew 26:51; John 18:10). Jesus, though, had rebuked Peter for fighting, submitting to His own arrest Matthew 26:52–54). By this time, Peter may have heard the report from inside the high priest's house that Jesus had been found guilty and condemned to die (Matthew 26:64–66).When people in the courtyard began to accuse Peter of being with Jesus, he recognized he was in real danger of being arrested and executed. Suddenly, his courage has failed, and he does not want to die. Before he can leave, though, he is approached one more time by people in the courtyard who recognize his accent is like that of the "Galilean" condemned to death (Matthew 26:73).
Peter begins to swear a curse on himself if he is lying. He once again vows on something sacred that he truly, really, honestly doesn't even know Jesus, at all. Following that third denial, the rooster crows signaling morning has come. Jesus' words have proved true, and Peter knows that he has proved faithless to his master, after all (Matthew 26:75).